Contention access procedures of ALOHA or Slotted ALOHA type are known. These procedures are based on random-access protocols. Each user accesses the transmission resources in an independent manner in relation to the other users. For each packet transmitted, the user waits for an acknowledgement of receipt from the recipient. If it does not receive it, it retransmits the same data with a random delay and this mechanism is repeated until receipt of an acknowledgement of receipt or until a maximum number of attempts have been made.
This type of procedure exhibits the drawback of significantly lengthening the lag in transmitting a packet when retransmissions are required, including at relatively low loading of the communication network (that is to say when there are few terminals with respect to the resource available) since there is no provision for a mechanism for substantially and preventively decreasing the initial rate of collisions between packets sent by distinct senders and therefore the retransmission rate, and this may give rise to appreciable lags.
So-called CRDSA (Contention Resolution Diversity Slotted ALOHA) contention access procedures are also known, such as described for example in the European patent published under the number EP 1 686 746 B1. For each packet to be sent, one or more replicas of this packet are generated and transmitted, simultaneously with the source packet and with a random transmission lag drawn initially from within a window corresponding to the logical frame following the arrival of the packet. This mechanism allows the receiver to reconstruct a packet that may possibly have been lost because of a collision by virtue of the redundancy introduced by way of the replicas sent and by virtue of collided packet interference cancellations if a packet or a replica originating from another sender were able to be decoded on another position in the frame.
The congestion control algorithm defined, for a so-called CRDSA procedure, in the European patent published under the number EP 1 686 746 B1 uses an item of information on the loading of the network. The way in which this network loading is evaluated is however not specified. An item of information on the loading of the network originating from a central entity is transmitted regularly to the various terminals. If this loading exceeds a threshold, each terminal will increase, according to a decreasing probability and in an independent manner, the transmission delay for the fragments that it must send by an additional logical frame. If the loading decreases below the threshold, the terminal will also decrease, according to a given probability, the delay in sending its fragments by a logical frame. Each terminal will therefore wait for a different number of logical frames as a function of the inherent evolution of its congestion window, knowing that a fragment and its replicas will be able to be transmitted only on a single logical frame. The expression logical frame is understood to mean a fixed-duration time interval defined by the standard or the system employed to communicate and which constitutes the temporal unit making it possible to fix a marker to the senders and to the receivers for the transmission and the reception of the data packets or the fragments of these packets. A logical frame can be composed of a given number of elementary physical frames or slots. In particular, in the case of contention access procedures which, for each fragment of useful data to be sent, generate one or more redundant fragments, such as for example the CRDSA procedure, all the fragments (useful and redundant) are transmitted in a logical frame.
The approach advocated with regard to congestion control in the European patent published under the number EP 1 686 746 B1, is an approach which reacts to the overloading of the transmission channel by using a notion of loading threshold and which is therefore not a preventive approach. A non-negligible number of collisions may thus occur temporarily at the moment at which the threshold is crossed and it may happen that the reaction time to this overload being too large, this gives rise to retransmissions of messages or message fragments giving rise to an increase in the message transmission lag. This approach also involves a policy for control of congestion and access to the distinct channel between terminals and which is not managed in a centralized manner, and this may potentially penalize certain terminals and favour others. This approach may potentially give rise to an inequity between the user terminals and makes it difficult to apply distinct quality-of-service levels for packets originating from one and the same terminal or different terminals. These procedures also exhibit the drawback of artificially increasing the loading of the network on account of the replicas systematically generated for each packet to be sent, thereby making it more difficult to evaluate the actual loading, stated otherwise the loading related to the useful data.